The present invention relates to photoinitiators which have visible light sensitivity and are useful for laser imaging systems. In general such imaging systems have much lower light intensity than conventional lithographic exposure apparatus. More particularly, the invention relates to a photopolymerizable composition having increased effective photosensitivity which find use in producing photosensitive elements such as lithographic printing plates of the projection speed variety.
The prior art teaches a variety of photopolymerizable compositions. These generally comprise an ethylenically unsaturated monomer or oligomer preferably having two or more double bonds, a photoinitiator capable of causing the free radical polymerization of the unsaturated component upon exposure to actinic radiation, and usually binder resins, colorants and components to prevent premature polymerization. Further, these compositions are widely used in photopolymer type photoresists, color proofing films and printing plates.
Useful photo polymerization initiators include various substances which have been found to be practical. For example, there are benzoin compounds such as benzoin, benzoin methyl ether, or benzoin ethyl ether, etc., carbonyl compounds such as benzil, benzophenone, acetophenone or Michler's ketone, etc., azo compounds such as azobisisobutyronitrile, as well as initiators of the acridine, quinoxaline or phenazine classes. While such photoinitiators are certainly useful within their concepts, it is often desired to produce photographic elements which are sensitive in the visible portion of the spectrum which have a greater effective photospeed than these known initiators. Higher effective photospeed is desired for photosensitive elements which are exposed by low intensity lasers or projection speed plates. In non-projection speed plates, exposure is conducted by placing an exposure mask in direct contact with the photographic emulsion in a vacuum frame. Exposure is then conducted with a high intensity ultraviolet light source. In a projection exposure system, the exposure mask is placed in the path of an exposure unit between the light source and a camera's objective lens. The image projects through the lens down onto a printing plate positioned on a platform. Exposure energy and duration is greatly reduced, therefore, photosensitivity should be commensurately enhanced. The foregoing classes of photo-initiators alone are not satisfactorily sensitive for such use. Lasers used for exposure are also of the low intensity variety. The invention provides a synergistic admixture of compounds which is capable of initiating polymerization upon exposure to light in the 400-500 nm range. It is useful for exposure with a 488 nm argon laser.